The present invention relates to a spread spectrum communications apparatus composed of a transmitter and a receiver conforming to a spread spectrum communication system, and a synchronization acquisition method therein.
Recently, since the Federal Communications Commission has released the ISM (industrial, scientific and medical) band for spread spectrum communications, products conforming to the spread spectrum communication system have been introduced in various communication appliance fields such as cordless telephones. The spread spectrum communication system features an excellent resistivity of interference and communication security. When employing this system, the FCC permits transmission at a larger output than in the conventional analog communication system. It is therefore an advantage of this system that the communication distance can be extended. On the other hand, products conforming to this system are generally based on the digital communication system, and are hence complicated and expensive as compared with the products of the conventional analog communication system. In such a background, by adding simple circuits for spreading and de-spreading to the conventional analog communications apparatus, an inexpensive spread spectrum communications apparatus is proposed. This spread spectrum communications apparatus based on the analog communication system has the following merits.
i) The frequency demodulator is higher in receiving sensitivity as compared with the demodulator of digital system, so that the communication distance can be extended.
ii) In sound transmitting system, in particular, since coding of sound is not needed, there is no delay of sound due to encoder.
An outline of a prior art of spread spectrum communications apparatus on the basis of analog communication system and its problems are explained below by referring to the drawings.
FIG. 8 is a block diagram showing a general conventional spread spectrum communications apparatus. A first communication device 1 and a second communication device 41 individually have the own transmitter and receiver, so that they can communicate with each other by wireless means. Information input into the first communication device 1 is modulated by an FM modulator 2, and is output as a narrow-band FM modulated signal 7 which is the same as in the conventional analog communication system. This signal 7 is added, together with a spreading code 8 generated in PN code generator 6, to a spreader 3, and modulated, and output as a spread spectrum modulated signal 9. This signal 9 is further converted to radio frequency in an RF transmitter 4 and then amplified to be an RF signal 10, which is released from a transmitting antenna 5. On the other hand, the RF signal transmitted from the first communication device 1 is received by a receiving antenna 51 of the second communication device 41, and is amplified in an RF receiver 52. An RF spread spectrum modulated signal 59 output from the RF receiver 52 and a de-spreading code 65 generated in a PN code generator 58 are mixed in a de-spreader 53. At this time, async circuit 57 controls the de-spreading code 65 so as to be synchronized with the RF spread spectrum modulated signal 59, and an original narrow-band FM modulated signal 60 is obtained as an output signal of the de-spreader 53. This signal 60 is converted into an intermediate frequency signal 61 in an IF section 54, and filtered, and demodulated in an FM demodulator 55 into an original information signal.
The sync circuit 57 is a kind of sliding correlator making use of the output voltage of Received Signal Strength Indicator (RSSI) of the IF section 54, and the correlation operation is as follows. The de-spreading code 65 used in de-spreading and the spreading code 8 in spreading in the first communication device 1 are exactly the same codes and exactly same in speed. Therefore, by sequentially changing the phase of the de-spreading code 65 for the RF spread spectrum modulated signal 59, and monitoring the output voltage 62 of the RSSI indicating the signal level after being converted and filtered into the intermediate frequency signal 61 by the IF section 54, it is known that a maximum voltage value is obtained when the phase is completely synchronized (when the correlation value of RF spread spectrum modulated signal 59 and de-spreading code 65 are maximum). It means that de-spreading is conducted by tracing and acquiring the phase for obtaining this maximum voltage while sliding the phase of the de-spreading code 65. In the configuration in FIG. 8, the output voltage 62 of RSSI is digitized by an ADC (analog-digital converter) 56, and in order to acquire and hold on the phase for maximizing this data value, the sync circuit 57 adjusts the phase of the PN (pseudo-noise) code generator 58 by a phase control signal 64 (hereinafter, the output voltage of the ADC is called RSSI data or RSSI voltage).
The sync circuit 57 has two operation modes, that is, sync acquisition and sync hold-on. First, the sync acquisition mode determines an approximate sync position of xc2xd chip unit according to the following procedure (1) to (3).
(1) The de-spreading code 65 is issued from the PN code generator 58 in a proper phase for a specific time, and the RSSI data 63 and the phase of the de-spreading code 65 at this time are recorded.
(2) Shifting the phase by xc2xd chip, the de-spreading code 65 is output for a specific time, and when the RSSI data 63 at this time is higher than the recorded value at step (1), the recorded value is updated.
(3) The same procedure is repeated for all phases in xc2xd chip unit, and jumping to the phase (sync point) corresponding to the maximum value of the obtained RSSI data 63, the sync acquisition is completed.
In the sync hold-on mode following the completion of sync acquisition, the phase is adjusted more finely (for example, xe2x85x9 chip) from the sync point obtained in the sync acquisition mode, and the phase is held on within xc2x1xe2x85x9 chip from the sync point.
In a block diagram of the sync circuit in FIG. 5, the sync circuit 57 includes a determining section 70, and a memory 71. FIG. 6 is a flowchart showing the first acquisition operation of the sync circuit. In FIG. 6, xe2x80x9cPHASExe2x80x9d is the count value showing sequential changes of the phase of the de-spreading code 65 (PHASE=1, 2, . . . , N, where N is an integer and denotes the number of times of detection of the correlation value determined by the number of chips/phase increment, that is, the number of times of sampling). Suppose the number of chips is 127. The RSSI is the RSSI voltage 63 (see FIG. 1) when the count value is xe2x80x9cPHASExe2x80x9d. xe2x80x9cPEAKxe2x80x9d is the maximum value of the detected RSSI. xe2x80x9cPEAKPHASExe2x80x9d is the count value when the RSSI is maximum.
First, the determining section 70 (see FIG. 5) sets initially at step S21 shown in FIG. 6, waits for a specific time (step S22), and samples the RSSI voltage every time the phase changes sequentially (step S23). Next, comparing if the presently sampled RSSI voltage has exceeded the hitherto maximum value or not (step S24), if exceeding, the RSSI is set as a new xe2x80x9cPEAKxe2x80x9d, and the count value at this time is set as new xe2x80x9cPEAKPHASExe2x80x9d (step S25). Then, judging if xe2x80x9cPHASExe2x80x9d has exceeded N (in this case, N is 127/(xc2xd)=254) or not (step S26), and if exceeding, the first acquisition operation is terminated, and if not exceeding, the phase is advanced by xc2xd chip (step S27). At step S24, if judged not to be RSSI greater than PEAK, the process jumps to step S26. Thus, by storing xe2x80x9cPEAKxe2x80x9d as the maximum value of RSSI voltage sampled so far, when all steps of first acquisition operation in FIG. 6 are over, the maximum value of the final RSSI voltage is stored as xe2x80x9cPEAKxe2x80x9d. When the first acquisition operation is complete, a second acquisition is operated in a range of 8 chips before and after xe2x80x9cPEAKPHASExe2x80x9d. At this time, by presetting the threshold level xe2x80x9cDECRSSIxe2x80x9d as shown in FIG. 7A and FIG. 7B, when the condition that the sampling value of the RSSI voltage in the second acquisition operation is lager than (PEAKxe2x88x92DECRSSI), it is judged that the sync acquisition is completed, and the operation moves to the sync hold-on mode.
In this method, however, in the absence of object signal, a maximum value caused by noises is obtained as shown in FIG. 7B. At this time, in the second acquisition operation, the sampling value of the RSSI voltage is the maximum value caused by noises. Its amplitude is about the same as that of PEAK, and this value exceeds (PEAKxe2x88x92DECRSSI) very often, thereby going to the sync hold-on mode easily. When moving thus to the sync hold-on mode by noises, the CPU 67 recognizes the asynchronous state by an out-of-sync detector 66, and sends out a reset command for returning to the sync acquisition mode. During this time, however, sync acquisition is not carried out, and hence it takes a longer time until the sync is established. Or, in the case of a cordless telephone based on the analog communication system, generally, data for monitoring the communication status is not exchanged, and it is hard to judge the asynchronous state completely during communication and it is also difficult to make an adequate reset. In this case, the data can be exchanged during communications, but the cost of the apparatus is increased and the circuit is complicated, and merits of analog communication system are sacrificed.
It is hence an object of the invention to present a spread spectrum communications apparatus heightened in the judging precision in sync acquisition and enhanced in performance, without increasing the number of parts or cost, and a synchronization acquisition method in the spread spectrum communications apparatus.
The spread spectrum communications apparatus of the invention has a receiver for receiving an RF spread spectrum modulated signal, and obtaining a primary modulated signal modulated by an information signal.
The receiver includes:
(a) an RF receiver for receiving an RF spread spectrum modulated signal,
(b) a de-spreader for generating an original primary modulated signal by using a de-spreading code from the RF spread spectrum modulated signal, and
(c) a sync circuit for performing synchronization acquisition operation composed of a first acquisition operation for synchronizing the received RF spread spectrum modulated signal with de-spreading code and a second acquisition operation following its completion, and detecting the correlation value of the RF spread spectrum signal and de-spreading code at every time of sequential change of phase value of the de-spreading code.
The sync circuit judges the maximum value of the correlation value from the correlation value for the number of times of detection in the first acquisition operation and the corresponding phase value, calculates the average of the correlation value for the number of times of detection, and compares to judge if the maximum value is larger than the threshold level defined by the sum of the average and the specified threshold level, and judges if the first acquisition operation is completed or not.
The synchronization acquisition method in the spread spectrum communications apparatus of the invention is as follows.
The synchronization acquisition method includes a first acquisition operation and a second acquisition operation following the completion of the first acquisition operation.
The first acquisition operation judges the maximum value of the =correlation value and the phase value corresponding to this maximum value from the correlation value for the number of times of detection of the correlation value determined in the relation of
number of times of detection=(number of chips)/(phase increment of de-spreading code per detection) 
and the corresponding phase value, and calculates the sum of the correlation values for the number of times of detection.
From the sum, the average of the correlation value is calculated, and it is compared if the following relation is established or not.
(maximum value of correlation value) greater than (average of correlation value)+(specified threshold level) 
On the basis of the result of comparison, it is judged if the first acquisition operation is over or not.
Thus, the invention obtains the spread spectrum communications apparatus and its synchronization acquisition method capable of realizing a high judging precision in synchronization acquisition, without increasing the number of parts or costs of the spread spectrum communications apparatus.